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© The Rockefeller University Press, 0021-9525/1998//355 $5.00
The Journal of Cell Biology, Volume 142, Number 2, , 1998 355-364


Articles

Signal Sequence Recognition in Cotranslational Translocation by Protein Components of the Endoplasmic Reticulum Membrane



Walther Mothes*, Berit Jungnickel*, Josef Brunner{ddagger}, and Tom A. Rapoport*

* Howard Hughes Medical Institute and Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115; and {ddagger} Laboratorium für Biochemie, Eidgenössishe Technische Hochschule Zürich, CH-8092 Zürich, Switzerland

We have investigated the role of membrane proteins and lipids during early phases of the cotranslational insertion of secretory proteins into the translocation channel of the endoplasmic reticulum (ER) membrane. We demonstrate that all steps, including the one during which signal sequence recognition occurs, can be reproduced with purified translocation components in detergent solution, in the absence of bulk lipids or a bilayer. Photocross-linking experiments with native membranes show that upon complete insertion into the channel signal sequences are both precisely positioned with respect to the protein components of the channel and contact lipids. Together, these results indicate that signal sequences are bound to a specific binding site at the interface between the channel and the surrounding lipids, and are recognized ultimately by protein–protein interactions. Our data also suggest that at least some signal sequences reach the binding site by transfer through the interior of the channel.

Key Words: signal sequence • endoplasmic reticulum • translocation • cross-linking • Sec61p



Abbreviations used in this paper: DBC, deoxyBigCHAP; ER, endoplasmic reticulum; RNC, ribosome–nascent chain complex; SRP, signal recognition particle; TRAM, translocating chain-associating membrane protein.

T.A. Rapoport is a Howard Hughes Medical Institute Investigator. The work was further supported by a grant from the National Institutes of Health (GM52586) to T.A. Rapoport and by the Swiss National Science Foundation to J. Brunner.

Drs. Mothes and Jungnickel contributed equally to this work.

Dr. Jungnickel's present address is Institut für Genetik, Universität Köln, Weyertal 121, 50931-Cologne, Germany.



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